Shaft furnace smelting of oxidic ores, concentrates or calcines

ABSTRACT

Method and apparatus for direct shaft furnace smelting of oxidic ores, concentrates or calcines, the method comprising causing a pelletized or briquetted mixture of the ore or concentrate or calcines and coal to gravitate slowly downwards first through one or more carbonizing columns having holes or slots in the walls thereof and then through a main furnace shaft therebeneath, introducing oxygen-containing gas into a combustion chamber surrounding or partly surrounding the carbonizing column or columns to effect substantial combustion of tars and combustible gases emerging through the holes or slots from the descending material in the carbonizing column or columns, withdrawing the products of combustion from the combustion chamber, and withdrawing the metal or alloy produced from the main furnace shaft; and apparatus adapted therefor.

United States Patent Worner [54] SHAFT FURNACE SMELTING OF OXIDIC ORES, CONCENTRATES OR CALCINES [72] Inventor: Howard K. Worner, North Balwyn, Vic-v toria, Australia [73] Assignee: Conzinc Riotinto of Australia Limited,

Melbourne, Victoria, Australia [22] Filed:

Oct. 6, 1969 [21] App]. No.: 864,051

[30] Foreign Application Priority Data Oct. 15, 1968 US. Cl

Australia ..44834/68 UNITED STATES PATENTS Tschemoff ..266/31 Dickey ..266/25 51 Mar. 28, 1972 FOREIGN PATENTS OR APPLICATIONS 132,654 5/1949 Australia ..266/25 12,637 6/1894 Great Britain ..266/25 Primary Examiner-Gerald A. Dost Attorney-Ryder & Hefter [5 7] ABSTRACT Method and apparatus for direct shaft furnace smelting of oxidic ores, concentrates or calcines, the method comprising causing a pelletized or briquetted mixture of the ore or concentrate or calcines and coal to gravitate slowly downwards first through one or more carbonizing columns having holes or slots in the walls thereof and then through a main furnace shaft therebeneath, introducing oxygen-containing gas into a combustion chamber surrounding or partly surrounding the carbonizing column or columns to effect substantial combustion of tars and combustible gases emerging through the holes or slots from the descending material in the carbonizing column or columns, withdrawing the products of combustion from the combustion chamber, and withdrawing the metal or alloy produced from the main furnace shaft; and apparatus adapted therefor.

15 Claims, 3 Drawing Figures SHEET 2 [IF 2 PATENTED MR 2 8 I972 SHAFT FURNACE SMELTING OF OXIDIC ORES, CONCENTRATES OR CALCINES This invention relates to the shaft furnace smelting of oxidic ores, concentrates or calcines.

In recent years several attempts have been made to by-pass coke ovens and other coking operations by charging directly to shaft furnaces composite pellets or briquettes containing coal as a major component. These direct smelting attempts have generally not been successful because of problems arising from a combination of a physical breakdown of the pellets or briquettes and the liberation of tarry matter which accumulated in the top of the shaft and gas offtakes ultimately forcing shut downs for clean out.

This invention makes possible the direct charging of composite pellets or briquettes containing caking coal and ensures that a. the pellets or briquettes remain strong during the handling into and the passage through carbonization stage, and

b. the tarry matter is liberated and combusted to a substantial degree in a heating-up open space within the top of the furnace and so does not foul-up the top of the shaft and gas offtaltes.

One advantage of the invention is that high cost low reactivity metallurgical coke, essential for good operation in conventional blast furnaces can be replaced by cheaper caking coal. The principal requirement of the coal used is that it should develop a high fluidity when passing through the early stages of pyrolysis. If the coal available does not possess this quality in sufficient degree, the blend of the coal with the oxidic fines can be supplemented with tar, bitumen or other liquid form of hydrocarbon that will increase the capacity of the coal material to diffuse amongst and wet" the particulate oxidic components of the pellets (or briquettes) during heating through the temperature range 320 to 480 C.

Another advantage of the invention is that it permits continuous charging of pellets or briquettes into shaft furnaces in such a way that complex sealing and double or triple bell and hopper systems are not required.

The invention is applicable principally to the direct shaft furnace smelting of oxide ores and concentrates, such as iron, tin and nickel bearing lateritic ores and concentrates. The invention is also applicable to the smelting of calcines or other oxidic metallurgical products. The product of smelting may be a liquid metal or alloy which is tapped from the crucible of the shaft furnace or a metal vapour which may be withdrawn from the upper end of the shaft. For example, in the case of zinc calcines and mixtures of zinc calcines and roasted lead concentrates the zinc vapour leaves the top of the shaft rather than collecting with the less volatile lead-rich phase and/or slag in the crucible of the bottom of the furnace. In other instances an oxidic furnace by-product is recovered out of the top of the shaft furnace.

The invention accordingly provides a method for the direct shaft furnace smelting of oxidic ores or concentrates or calcines which comprises causing a pelletised or briquetted mixture of the ore or concentrate or calcines and coal to gravitate slowly downwards first through one or more carbonizing columns having holes or slots in the walls thereof and then through a main furnace shaft therebeneath, introducing oxygen-containing gas into a combustion chamber surrounding or partly surrounding the carbonizing column or columns to effect substantial combustion of tars and combustible gases emerging through the holes or slots from the descending material in the carbonizing column or columns, withdrawing the products of combustion from the combustion chamber, and withdrawing the metal or alloy produced from the main furnace shaft.

In one preferred form the invention consists in a method of continuous integrated feeding, carbonizing and smelting of oxidic ores or concentrates or calcines in a shaft furnace involving the following steps:

The oxidic ore fines or concentrates or calcines are first blended with finely ground caking coal, and if desired other additives, at least one of which is a binder capable of forming a bond at near ambient temperatures, pelletizing or briquetting the blend, allowing the pellets or briquettes to harden by action of the binder or binders, charging the hardened but as yet uncoked pellets or briquettes via an appropriate hopper or hoppers into one or more substantially vertical hollow columns, having holes or slots in the walls thereof to allow escape of the tars and other gaseous hydrocarbons released from the coal component in the pellets or briquettes as they pass through the pyrolysis temperature range while moving slowly down the columns. On emergence from the columns the pellets or briquettes are in a semi-coked condition and descend further down the shaft furnace until smelting is completed.

An oxygen-containing gas is blown via one or more ports or jets into the chamber or space surrounding the columns is order to achieve combustion of a substantial proportion of the tars and hydrocarbon gases emerging through the holes or slots and also part at least of the carbon monoxide and hydrogen emerging from the top of the burden in the furnace stack. The combustion of these tars and gases in the space around the hollow columns provides ample heat to ensure autogenous coking of the coal matter in the pellets or briquettes as they slowly descend within the hollow columns. The pellets or briquettes are heated by a combination of radiation, conduction and convection within the columns and emerge onto the top of the furnace stack burden at temperatures within the range 450 to 800 C. preferably within the range 550 to 750 C. and between 700 to 800 C. in the caseof zinc smelting. The hot combusted gases leave the space or chamber around the hollow columns via one or more appropriately located vents to pass via heat exchangers and gas cleaners to exhaust fans. High thermal efl'iciency may be secured by using these gases for various heating operations in metallurgical plants, such as for instance, the preheating of the air blown into the furnace via tuyeres or ports, moderate preheating of pellets or briquettes prior to charging into the hoppers at the top of the shaft furnace.

In another form of the invention the tars and other volatile pyrolysis products are substantially combusted by reaction with oxygen-containing gases in a combustion chamber surrounding the column or columns and mounted above but substantially separate from the smelting shaft proper; the substantially devolatilized and partially coked pellets or briquettes emerge from the slotted column or columns finally into the smelting shaft in which rapid smelting to a metal vapour, slag and possibly another less volatile metal-rich phase takes place.

For example, in the case of smelting zinc calcines the zinc leaves the top of the furnace entrained in a hot COCO,N gas mixture and enters a condenser for its recovery. If lead is present lead bullion is produced as well as slag. The tars produced, being substantially combusted in the top separate chamber, do not foul-up the top of the shaft and gas ofi'takes nor cause the reversion reactions shown below with the zinc vapour emerging from the top of the burden in the shaft proper:

Zn +CO, ZnO+CO However, if the pellets or briquettes have not reached a temperature of about 700 C. when they enter the shaft smelting zone proper there may be too much volatile matter left in the semi-coke component of the hot pellets or briquettes and the hydrogen, hydrocarbons and any H O derived therefrom can enter into undesirable back reactions with zinc vapour in the condenser. If on the other hand the pellets or briquettes reach a temperature above about 800 C. while still in the carbonizing column or columns, zinc vapour may begin distilling therefrom and the zinc oxide formed from this vapour in the exit gases will then have to be collected and recycled from the equipment used to clean The invention also provides apparatus for the direct shaft furnace smelting of oxidic ores or concentrates or calcines which comprises one or more hollow carbonizing columns disposed above a main furnace shaft, a combustion chamber or space surrounding or partly surrounding the carbonizing column or columns, each carbonizing column having holes or slots in the wall or walls thereof, means for feeding a pelletised or briquetted mixture of ore or concentrate and coal to the upper ends of the carbonizing column or columns, so that the pellets or briquettes descend slowly under gravity first through the carbonizing column or columns and then through the main furnace shaft, means for introducing oxygen-containing gas into the combustion chamber or space, and means for withdrawing metal or alloy from the main furnace shaft.

In another preferred form the invention consists in apparatus for achieving semi-coking of the coal component in composite pellets or briquettes of oxidic ores, concentrates or calcines comprising substantially vertical hollow columns made of heat resisting alloy or refractory ceramic arranged within the space or chamber above the main shaft of the shaft furnace, the space or chamber being substantially sealed except for one or more gas offtakes, each column having holes or slots, of such size or arrangement that the pellets or briquettes will not be able to fall through them, and extending for at least the length of the bottom half of each column. The columns permit the transfer of heat to the descending pellets or briquettes from the surrounding space in which the tars and gaseous hydrocarbons, plus any CO or I'I arising from the stack, are combusted by an oxygen-containing gas blown in via appropriately located ports or jets in the outer wall of the chamber or space. The columns are of such length, cross-sectional area and wall thickness as to permit the progressive heating of the pellets or briquettes descending therein to a temperature within the range 450 to 800 C. preferably within the range 550 to 750 C. and preferably between 700800C. in zinc smelting before they emerge.

In one form of the invention the combustion chamber is preferably removable from the main furnace shaft.

The oxygen-containing gas blown into the combustion space or chamber surrounding the carbonizing columns enters with sufficient velocity and pressure to ensure ample circulation and movement of flames and hot gases within it. Baftles or other means may also be employed within the combustion space to assist uniform heating of each of the columns, as otherwise varying degrees of coking may result in the several columns.

The vent or vents via which the hot combusted gases leave the combustion space or chamber surrounding the carbonizing columns are connected to hot gas mains which convey the hot gases to heat exchangers, gas cleaners and exhaust fans. Usually the combustion space or chamber surrounding the carbonizing columns is maintained at a slightly negative pressure in order to help draw the tarry matter and other hydrocarbons away from the pellets or briquettes through the slots or holes and into the space or chamber where combustion takes place. 7

The invention will be better understood by reference to the schematic diagrams shown in FIGS. 1, 2 and 3 of the accompanying drawings. Identical numbers in both figures have the same connotations.

It should be noted that these diagrams show typical simple forms of the invention only and are not intended to limit the shape, construction, and dimensions of any shaft furnace to which the invention may be applied, nor the number, configuration and size of the hollow columns used. In every application the relevant parameters will be selected to obtain maximum technical and economic efficiency of operation, in other words, optimum combination and devolatilization, coking and smelting.

In FIG. 1 a shaft furnace 10 is shown in the upper part of which three perforated carbonizing columns 11, 12, 13 are located. Preferably the columns 11, 12, 13 increase slightly and gradually in cross-sectional area from top to bottom. Composite pellets or briquettes 14 are fed into the upper ends of the columns 11, 12, 13 via hoppers 15. Two inlets 16a, 16b for oxygen-containing gases and one ofitake 18 for the products of combustion are provided. Said inlets 16, 16a are placed along the sides of the combustion space or chamber 19 surrounding the carbonizing columns 11, 12, 13 in such a way so as to ensure optimum movement and distribution of the hot oxygen-containing gases. The offtake 18 is connected to the upper end of the combustion chamber 19.

Slots or holes 20 are formed in the walls of the columns 11, 12, 13. The said slots or holes 20 are of such size or arrangement that the pellets or briquettes cannot fall out through said slots or holes. Tuyeres 21 are provided for the admission of oxygen-containing gas (e.g. air) at the bottom of the furnace. The melt collects in the crucible 22 and is tapped at outlet 23.

FIGS. 2 and 3 are similar to FIG. 1 but embody modifications which are particularly suited to smelting processes in which volatile metal products are formed. Accordingly provisions are made for the condensation of such metal vapours by having vapour condensers 24 connected to the main shaft of the furnace 10. The combustion chambers 19 are shown as separate removable units, that shown in FIG. 2 being of substantially rectangular vertical cross-section. Additional inlets 17 for oxygen-containing gases are located below the chamber 19.

The apparatus shown in FIG. 3 includes a waisted section 25 below the chamber 19 which serves to aid the flow of the volatile metal product into the vapour condenser 24 and also serves to direct the devolatilized-carbonized pellets or briquettes towards the center of the furnace.

Means (not shown) may, if desired, be provided for controlled rotation or other actuation of the hollow carbonizing columns 11, 12, 13 in any of the forms of the invention to accelerate or case the descent of the pellets or briquettes in said columns.

Reference will now be made to Examples of the practical application of the invention:

EXAMPLE 1 Smelting of Rich Hematite Ore Fines Ore of the following composition:

Fe 68.0 SiO, 2.8 Alp, 1.2 Loss on ignition 0.4 Balance 27.6 (mostly oxygen) was ball milled until 78 percent passed through a 320-mesh BS screen.

The ore fines were blended thoroughly witha. Caking coal of high Gieseler fluidity index and a chemical composition (on dry basis) of:

Fixed carbon 50.5

Volatiles 43.9 Ash 5.] (mostly SiO, and

z i) Sulphur 0.5

Ore fines 50 Coal 44 Lime 5 Cement The mixture was pelletized in a disc pelletizer to produce pellets ranging from one-half inch to seven-eigths inch diameter. The pellets 14 were allowed to dry out slowly over 24 to 40 hours and were hard enough to withstand the handling into the hoppers 15 at the top of a low shaft smelting furnace of the general type shown in FIG. 1. About 3 percent extra lump limestone (plus one-fourth in. minus seven-eighths in.) was added to maintain reasonably high basicity in the slags.

The experimental furnace had a total internal shaft height of 8 ft. with a shaft diameter of 18 inches and a hearth diameter of 12 inches. The vertically slotted carbonizing steel columns 11, 12, 13 were 3 feet long and the bed height in the shaft proper 24 was approximately 4 feet 6 inches.

The blast air was preheated to between 380 C. and 400 C. and blown in via six tuyeres 21. The air blown in at lower pressure via ports 16 and 17 was preheated to only 200 C.

The chemical composition of the pig iron tapped periodically from the crucible of the furnace at 23 averaged:

e Balance Sn 23.8 Fe 15.8 (mostly magnetite) SiO, l7.0 Al,0, 7.l CaO 5.0 MnO 1.0 S 1.7 ignition loss l 1.5 (including H,O at 900" C. and C0,) Balance 17.] (mostly oxygen) and b. A strongly caking coal assaying: (as used in Example 1) Fixed carbon 50.5 Volatiles 43.9 Ash 5.] Sulphur 0.5

and

c. Dry slaked lime (96% Ca(O1-l),, as binder and flux), and

d. Portland cement (as binder and flux).

The above materials (a), (b), (c) and (d) were thoroughly blended in the following proportions a. Concentrates 57 b. Coal 33 c. Lime 8 d. Cement 2 The mixture was pelletized in a disc pelletizer to produce pellets ranging in size from one-half inch to seven-eighths inch diameter. The pellets 14 were allowed to air dry over 24 to 30 hours by which time they had developed sufficient strength to permit their handling without serious degradation into the top hoppers of the low shaft furnace 10.

The furnace 10 used was the same as employed for Example 1 and the blast preheat temperature at 21 was the same, namely 380 to 400 C. However, the air blown in at inlets l6 and 17 was only at C. This was necessary to keep the temperature at the top of the burden in the shaft below 700 as otherwise excessive fume loss occurred. The partially carbonized pellets emerged from the carbonizing columns 11, 12, 13 at a preferred just dull red heat (550 to 650 C.).

Smelting proceeded smoothly with production of tin-iron alloy of the following average chemical composition:

k Sn 62.0 Fe 37.8 Other elements 0.2

This molten alloy is suitable for continuous refining in a WORCRA type furnace with countercurrent flow of slag relative to that of the molten alloy as described in the process of our US. Pat. No. 3,326,672.

The calcium-aluminium silicate slags assayed 0.5% to 0.8% Sn with between 5% and 8% Fe in the form of FeO.

The fume collected in a hot cyclone followed by a cooler and bag house assayed between 45% and 55% Sn as oxide. In later smelting campaigns this fume was blended in with the composite pellet mix thereby adding strength to the dry uncarboriized pellets 14.

EXAMPLE 3 Smelting of a 50:50 Mixture of Nickeliferous Laterite and Gamieritic Weathered Serpentine Ore The ore mixture contained the following percentages of the more significant components on a dry basis:

Ni 1.7 Fe 35.3 Cr 1.7 SiO, l7.0 MgO 7.0 ALO, 5.5 Mno 1.25

Balance mostly oxygen and H 0 The ores were dried at approximately 200 C. in a rotary kiln to drive off uncombined water and the dried material was hammer milled to a product such that 100 percent passed a 60-mesh BS screen and approximately 68 percent passed a 300-mesh BS screen. The ore mixture fines were then blended with hammer milled caking coal (the same as in Examples 1 and 2) and dry slaked lime in the following proportions:

Ore mixture 52 Coal 40 Lime 5 Limestone 3 This blend was pelletized in a disc pelletizer to produce pellets ranging in diameter from five-eighths inch to l inch. The pellets 14 were allowed to air dry over a period of 24 to 48 hours and ,were then hard enough to withstand handling associated with charging into the same shaft furnace used in Examples l and 2.

The blast preheat temperature at 21 was 375 to 400 C. and the air blown in via inlets 16 and 17 was at approximately 250 C.

The alloy tapped at intervals via taphole 23 was of the following average chemical composition:

1: Ni 6.2 Cr 3.l Co 0.3 C 2.2 Si 3.5

Mn 0.3 S 0.2 P 0.1 l Fe Balance The calcium-magnesium-iron-aluminium silicate slag contained 0.06% Ni and 0.15% Cr.

Over 95 percent of the nickel and approximately 80 percent of the iron were recovered in the alloy phase.

EXAMPLE 4 Smelting of Zinc Calcines Composite pellets were produced from: w

a. Zinc calcines produced by calcining Broken Hill zinc sulphide concentrates in a fluid bed roaster. The calcines had an average assay of:

S (total) 1.0 Pb 1.0

sio, 2.9

CaO 0.9

Other oxides 3.5

Balance 2|.5 (mostly oxygen) and b. Hammer-milled Richmond Main Coal from Kurri Kurri,

New South Wales, with an assay of:

Fixed carbon SL5 Volatile matter 42.4 Ash 4.6 Moisture 1.5 Sulphur 0.8

(This is a strongly caking coal with a Gieseler plastorneter reading of I730 Dial divisions per minute at 404 C.).

and

c. Dry slaked lime (plus 95% Ca(OH),). and

d. Portland Cement. I

The above materials (a), (b), (c) and (d) were blended in a paddle mixer in the following proportions:

a. Calcines 55 b. coal 40 c. lime 4 d. Portland cement l and pelletized in a disc type pelletizer. The pellets were ssinch to l-inch diameter in size. They were allowed to air-dry over 20 to 30 hours and were then charged into the hoppers of an integral carbonizer-smelting shaft furnace of the type shown in FIG. 2.

The experimental furnace used had a total shaft height of 8 feet and a diameter of 18 inches with a crucible hearth diameter of 12 inches. Six tuyeres 21 of heat resisting alloy were used. The carbonizing columns 11, 12, 13 were 3 ft. 6 in. long, and the heat resistant alloy waist 25 directed the hot devolatilized pellets down into the shaft 10. The pellets emerged from the bottom of the slotted columns 11, 12, 13 at 650 to 700 C. and were slightly hotter, 700 to 750 C., as they entered into the top of the shaft proper.

The zinc vapour emerging from the top of the hot pellet burden was carried off into a lead splash condenser of the type developed by Imperial Smelting Co. Ltd., England. CO/CO, ratios at the top of the burden varied between 3:1 and 4:1. Some actual analyses were:

lb 91: CO 22.6 23.2 14.1 CO, 7.2 6.2 5.5

The calcium-iron-aluminium silicate slags tapped from the crucible were generally similar to those tapped from ISF furnaces. Zinc contents ranged from 5.0 to 9.6 percent.

Some lead bullion also collected in the crucible and was tapped, with the slag, at intervals; the bullion settled from the latter in a forehearth outside the furnace.

I claim:

1. Apparatus for the direct shaft furnace smelting of oxidic ores or concentrates or calcines which comprises one or more hollow carbonizing columns disposed above a main furnace shaft, a combustion chamber or space surrounding or partly surrounding the carbonizing column or columns, each carbonizing column having holes or slots in the wall or walls thereof, means for feeding a pelletized or briquetted mixture of ore or concentrate or calcine and coal to the upper ends of the carbonizing column or columns, so that the pellets or briquettes descend slowly under gravity first through the carbonizing column or columns and then through the main furnace shaft, means for introducing oxygen-containing gas into the combustion chamber or space, and means for withdrawing metal or alloy from the main furnace shaft.

2. Apparatus according to claim 1 wherein means are provided for withdrawing molten metal or alloy from the crucible of the shaft furnace.

3. Apparatus according to claim 1 wherein means are provided for withdrawing a metal vapour from the upper end of the main shaft of the shaft furnace.

4. Apparatus according to claim 3 and having a condenser connected to the metal vapour outlet from the shaft furnace.

5. Apparatus according to claim 4 wherein the combustion chamber is waisted to aid the flow of metal vapour into the condenser and to direct the pellets towards the center of the main shaft of the shaft furnace.

6. Apparatus according to claim 1 wherein the holes or slots extend over at least the lower half of each carbonizing column.

7. Apparatus according to claim 1 wherein the holes or slots are configured such that the pellets or briquettes are not able to pass through them.

8. Apparatus according to claim 1 wherein the cross-sectional area of each carbonizing column increases gradually from its upper to its lower end.

9. Apparatus according to claim 1 wherein the carbonizing columns are formed of heat resistant alloy or refractory ceramic.

10. Apparatus according to claim 1 and having a gas ofitake connected to the combustion chamber.

11. Apparatus according to claim 1 and having a hopper connected to the upper end of each carbonizing column.

12. Apparatus according to claim 1 wherein a plurality of carbonizing columns is provided and the combustion chamber extends between and around the carbonizing columns.

13. Apparatus according to claim 1 wherein the combustion chamber is substantially removable from the main furnace shaft.

14. Apparatus according to claim 1 and having means for maintaining the combustion chamber at slightly negative pressure.

15. Apparatus according to claim 1 and having tuyeres for admitting oxygen-containing gas to the bottom of the shaft furnace.

* 1 i i i UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 a 652 a 069 Dated March 28, 1972 Inventor(s) Howard Knox Worner It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 2 Line 16 change "is" to "in" Column 2, Line 58 change 'Z," to "Zn" Column 5, Line 7 change "eigths" to "eighths" Column 5, Line 69 following the word "cement", numeral "2" should'appear under percentage 7 column Column 7, Line- 30 change "1730" to "2730" Signed and sealed this 10th day of October 1972'.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. ROBERT GOTTSCHALK r Attesting Officer Commissioner of Patents FORM PC4050 (10439) USCOMM-DC 60376-5 69 U.S. GDVERNMENT FRINTING OFFICE 1 I959 O-366'33l 

2. Apparatus according to claim 1 wherein means are provided for withdrawing molten metal or alloy from the crucible of the shaft furnace.
 3. Apparatus according to claim 1 wherein means are provided for withdrawing a metal vapour from the upper end of the main shaft of the shaft furnace.
 4. Apparatus according to claim 3 and having a condenser connected to the metal vapour outlet from the shaft furnace.
 5. Apparatus according to claim 4 wherein the combustion chamber is waisted to aid the flow of metal vapour into the condenser and to direct the pellets towards the center of the main shaft of the shaft furnace.
 6. Apparatus according to claim 1 wherein the holes or slots extend over at least the lower half of each carbonizing column.
 7. Apparatus according to claim 1 wherein the holes or slots are configured such that the pellets or briquettes are not able to pass through them.
 8. Apparatus according to claim 1 wherein the cross-sectional area of each carbonizing column increases gradually from its upper to its lower end.
 9. Apparatus according to claim 1 wherein the carbonizing columns are formed of heat resistant alloy or refractory ceramic.
 10. Apparatus according to claim 1 and having a gas offtake connected to the combustion chamber.
 11. Apparatus according to claim 1 and having a hopper connected to the upper end of each carbonizing column.
 12. Apparatus according to claim 1 wherein a plurality of carbonizing columns is provided and the combustion chamber extends between and around the carbonizing columns.
 13. Apparatus according to claim 1 wherein the combustion chamber is substantially removable from the main furnace shaft.
 14. Apparatus according to claim 1 and having means for maintaining the combustion chamber at slightly negative pressure.
 15. Apparatus according to claim 1 and having tuyeres for admitting oxygen-containing gas to the bottom of the shaft furnace. 